Perpendicular magnetic recording medium and magnetic recording/reproducing apparatus

ABSTRACT

According to one embodiment, disclosed is a perpendicular magnetic recording medium in which a magnetic recording layer has a stacked structure including a hard magnetic recording layer and soft magnetic recording layer each having magnetic crystal grains and a grain boundary region. The magnetic crystal grains in the hard magnetic recording layer contain Co and Pt, have the hcp structure, and are orientated in the (0001) plane. The magnetic recording layer has a residual squareness ratio of 0.95 or less and an irreversible reversal magnetic field of 0 Oe or less on a magnetization curve when a magnetic field perpendicular to the substrate surface is applied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2006-324994, filed Nov. 30, 2006, theentire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

One embodiment of the invention relates to a perpendicular magneticrecording medium and magnetic recording/reproducing apparatus to be usedin, e.g., a hard disk drive using the magnetic recording technique.

2. Description of the Related Art

Magnetic storage devices (HDDs) mainly used in computers to record andreproduce information are recently beginning to be used in variousapplications because they have large capacities, inexpensiveness, highdata access speeds, high data storage reliability, and the like, andthey are now used in various fields such as household video decks, audioapparatuses, and automobile navigation systems. As the range ofapplications of the HDDs extends, demands for large capacities increase,and high-density HDDs are more and more extensively developed in recentyears.

Presently commercially available magnetic recording/reproducingapparatuses use the longitudinal magnetic recording method. In thismethod, magnetic crystal grains forming a perpendicular magneticrecording layer for recording information have the easy magnetizationaxis parallel to a substrate. The easy magnetization axis is an axis inthe direction of which magnetization easily points. In the case of aCo-based alloy, the easy magnetization axis is a direction parallel tothe normal of the (0001) plane of the hexagonal close-packed structure(hcp) of Co. When recording bits of a longitudinal magnetic recordingmedium are downsized in order to increase the recording density, themagnetization reversal unit diameter of the magnetic layer may becometoo small, and the thermal decay effect that thermally erasesinformation in the magnetic layer may worsen the recording/reproductioncharacteristics. In addition, as the density increases, the influence ofa demagnetizing field generated in the boundary region between therecording bits often increases noise produced from the medium.

By contrast, in a so-called perpendicular magnetic recording method inwhich the easy magnetization axis in a perpendicular magnetic recordinglayer is almost perpendicular to the substrate, the influence of thedemagnetizing field between the recording bits is small and the mediumis magnetostatically stable even when the density increases. Therefore,the perpendicular magnetic recording method is recently attracting agood deal of attention as a technique that replaces the longitudinalrecording method. The perpendicular magnetic recording medium generallycomprises a substrate, an orientation control underlayer that orientatesmagnetic crystal grains in a perpendicular magnetic recording layer inthe (0001) plane and also reduces the orientation dispersion, theperpendicular magnetic recording layer containing a hard magneticmaterial, and a protective layer that protects the surface of theperpendicular magnetic recording layer. In addition, a soft magneticunderlayer that concentrates magnetic fluxes generated from a magnetichead during recording is formed between the substrate and orientationcontrol underlayer.

To increase the recording density of even the perpendicular magneticrecording medium, it is necessary to reduce noise while maintaining thethermal stability. A method generally used as the noise reduction methodis to reduce the magnetic interaction between the magnetic crystalgrains in the recording layer by magnetically isolating the magneticcrystal grains, and downsize the magnetic crystal grains themselves atthe same time. It is disclosed by, for example, Jpn. Pat. Appln. KOKAIPublication No. 2002-83411 has disclosed a method of forming aperpendicular magnetic recording layer having a so-called granularstructure by adding SiO₂ and the like to the recording layer so as tosurround magnetic crystal grains with a grain boundary region mainlycontaining these additives.

On the other hand, when reducing noise by the method as described above,it is inevitably necessary to increase the magnetic anisotropic energyof the magnetic crystal grains in order to ensure the thermal stability.If the magnetic anisotropic energy of the magnetic crystal grains isincreased, however, the anisotropic magnetic field, saturation magneticfield, and coercive force also increase. Since this increases therecording magnetic field necessary for magnetization reversal for datawrite as well, the writability of a recording head decreases. As aconsequence, the recording/reproduction characteristics deteriorate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A general architecture that implements the various feature of theinvention will now be described with reference to the drawings. Thedrawings and the associated descriptions are provided to illustrateembodiments of the invention and not to limit the scope of theinvention.

FIG. 1 is a sectional view showing an example of a perpendicularmagnetic recording medium according to an embodiment of the invention;

FIG. 2 is a graph showing magnetization curves of an example of acomparative perpendicular magnetic recording medium;

FIG. 3 is a graph showing magnetization curves of the example of theperpendicular magnetic recording medium according to the embodiment;

FIG. 4 is a graph showing schematic magnetization curves for explainingone definition of a term according to one embodiment of the invention;

FIG. 5 is a sectional view showing the perpendicular magnetic recordingmedium according to a second embodiment of the invention;

FIG. 6 is a partially exploded perspective view showing a magneticrecording/reproducing apparatus according to one embodiment of theinvention;

FIG. 7 is a view schematically showing the section of a perpendicularmagnetic recording medium according to an embodiment of the invention;

FIG. 8 is a graph showing the relationship between the soft magneticrecording layer film thickness and a residual squareness ratio Rs of theperpendicular magnetic recording medium according to the embodiment ofthe invention;

FIG. 9 is a graph showing the relationship between the soft magneticrecording layer film thickness and an irreversible reversal magneticfield Hi of the perpendicular magnetic recording medium according to theembodiment of the invention;

FIG. 10 is a graph showing the relationship between the soft magneticrecording layer film thickness and a coercive force Hc of theperpendicular magnetic recording medium according to the embodiment ofthe invention;

FIG. 11 is a graph showing the relationship between the soft magneticrecording layer film thickness and the SNR of the perpendicular magneticrecording medium according to the embodiment of the invention;

FIG. 12 is a graph showing the relationship between the soft magneticrecording layer film thickness and the OW of the perpendicular magneticrecording medium according to the embodiment of the invention;

FIG. 13 is a graph showing the relationship between the soft magneticrecording layer film thickness and the V1000/V0 of the perpendicularmagnetic recording medium according to the embodiment of the invention;

FIG. 14 is a graph showing the relationship between the Co compositionof a CoNi SiO₂ soft magnetic recording layer and the medium SNR;

FIG. 15 is a graph showing the relationship between the Co compositionof the CoNi SiO₂ soft magnetic recording layer and the OW;

FIG. 16 is a graph showing the relationship between the Co compositionof the CoNi SiO₂ soft magnetic recording layer and the thermal decayresistance;

FIG. 17 is a graph showing the relationship between the Pd interlayerfilm thickness and the residual squareness ratio Rs;

FIG. 18 is a graph showing the relationship between the Pd interlayerfilm thickness and the irreversible reversal magnetic field Hi;

FIG. 19 is a graph showing the relationship between the Pd interlayerfilm thickness and the coercive force Hc;

FIG. 20 is a graph showing the relationship between the Pd interlayerfilm thickness and the medium SNR;

FIG. 21 is a graph showing the relationship between the Pd interlayerfilm thickness and the OW; and

FIG. 22 is a graph showing the relationship between the Pd interlayerfilm thickness and the thermal decay resistance.

DETAILED DESCRIPTION

Various embodiments according to the invention will be describedhereinafter with reference to the accompanying drawings. In general,according to one embodiment of the invention, a perpendicular magneticrecording medium of the present invention basically comprises asubstrate, and a soft magnetic underlayer, nonmagnetic underlayer, andmagnetic recording layer sequentially stacked on the substrate. Thismagnetic recording layer is a multilayered magnetic recording layerhaving a stacked structure of a hard magnetic recording layer and softmagnetic recording layer. Also, each of the hard magnetic recordinglayer and soft magnetic recording layers has magnetic crystal grains anda grain boundary region surrounding the magnetic crystal grains. Inaddition, the magnetic crystal grains in the hard magnetic recordinglayer contain Co and Pt, has the hcp structure, and are orientated inthe (0001) plane. Furthermore, this magnetic recording layer has aresidual squareness ratio of 0.95 or less and an irreversible reversalmagnetic field of 0 Oe or less on a magnetization curve when a magneticfield perpendicular to the substrate surface is applied.

A magnetic recording/reproducing apparatus of the present invention is amagnetic recording/reproducing apparatus to which the perpendicularmagnetic recording medium of the present invention is applicable, andhas the perpendicular magnetic recording medium and arecording/reproducing head.

The present invention provides a perpendicular magnetic recording mediumhaving a high medium SNR, a good overwrite (OW) characteristic, and ahigh thermal decay resistance, and makes high-density recordingfeasible.

The present invention uses the soft magnetic recording layer and hardmagnetic recording layer each having the magnetic crystal grains andgrain boundary region as the recording layer. The soft magneticrecording layer has a weak coercive force and small saturation magneticfield, and the hard magnetic recording layer has a high thermal decayresistance. The present invention also uses a stacked soft magneticrecording layer structure. When a magnetic field perpendicular to thesubstrate surface is applied, the residual squareness ratio is 0.95 orless, and the irreversible reversal magnetic field is 0 Oe or less. Thismakes it possible to reduce the medium noise, improve the writability,and increase the thermal decay resistance at the same time.

This is so presumably because when the exchange coupling force ismoderately exerted between the magnetic crystal grains in the softmagnetic recording layer and the magnetic crystal grains in the hardmagnetic recording layer, the two layers do not completely coherentlyreverse magnetization; prior to magnetization reversal in the hardmagnetic recording layer, the soft magnetic recording layer can startreversible magnetization rotation before the applied magnetic fieldreaches the reversal magnetic field in the hard magnetic recordinglayer.

FIG. 1 is a sectional view showing an example of the perpendicularmagnetic recording medium according to the present invention.

As shown in FIG. 1, a perpendicular magnetic recording medium 10 has astructure in which a soft magnetic underlayer 2, nonmagnetic underlayer3, and perpendicular magnetic recording layer 4 are sequentially stackedon a substrate 1. The perpendicular magnetic recording layer 4 has twolayers, i.e., a hard magnetic recording layer 4-1 and soft magneticrecording layer 4-2.

In the magnetic recording layer of the perpendicular magnetic recordingmedium of the present invention, the hard magnetic recording layer canbe a single layer or a stacked structure of two or more layers. The softmagnetic recording layer can also be a single layer or a stackedstructure of two or more layers.

The hard magnetic recording layer of the perpendicular magneticrecording medium of the present invention has the granular structure inwhich the nonmagnetic grain boundary region surrounds the magneticcrystal grains.

As the magnetic crystal grain material used in the hard magneticrecording layer, it is possible to use an alloy material practicallyorientated in the (0001) plane, containing Co and Pt, and having the hcpstructure. When Co alloy crystal grains having the hcp structure areorientated in the (0001) plane, the easy magnetization axis readilypoints in a direction perpendicular to the substrate surface. It is alsopossible to use, e.g., a Co—Pt-based alloy material or Co—Pt—Cr-basedalloy material. These alloys have high crystal magnetic anisotropicenergy, and hence increase the thermal decay resistance of the magneticrecording medium. To improve the magnetic characteristics, at least oneadditive element selected from the group consisting of Ta, Cu, B, and Ndcan be added to these alloy materials if necessary.

Whether the magnetic recording layer has the granular structure can beconfirmed by observing the magnetic recording layer surface by using,e.g., a transmission electron microscope (TEM). When energy dispersionX-ray analysis (EDX) is used together, it is possible to determine theelements in the magnetic crystal grains and grain boundary region, andevaluate the compositions of these elements.

The orientation plane of the magnetic crystal grains in each layer canbe evaluated by the θ-2θ method by using, e.g., a general X-raydiffraction apparatus (XRD).

The soft magnetic recording layer used in the present invention has thegranular structure similar to the hard magnetic recording layerdescribed above.

The soft magnetic recording layer and hard magnetic recording layerhaving the granular structure are used to form the nonmagnetic grainboundary region around the magnetic crystal grains in the magneticrecording layer, thereby reducing the exchange interaction between themagnetic crystal grains. This makes it possible to reduce the transitionnoise in the recording/reproduction characteristics.

In the present invention, a layer in which the saturation magnetizationamount is larger than that of the hard magnetic recording layer and themagnetic anisotropic energy perpendicular to the film surface is lowerthan that of the hard magnetic recording layer can be used as the softmagnetic recording layer. More specifically, it is possible to use alayer in which the saturation magnetization amount is 700 to 1,700emu/cc and the magnetic anisotropic energy perpendicular to the filmsurface is 2×10⁶ erg/cc or less. As the magnetic crystal grain materialof the soft magnetic recording layer meeting the conditions, Fe, Co, oran alloy containing 35% or more of Fe or Co can be used. Examples areFe—Co, Fe—Ni, and Co—Ni alloys. A Co—Ni alloy is favorable from theviewpoint of the oxidation resistance. In this case, the Co compositionin the Co—Ni alloy can be 45 to 80 at. %.

A compound such as an oxide, nitride, or carbide can be used as thematerial forming the grain boundary region of the hard magneticrecording layer and soft magnetic recording layer. These compoundsreadily deposit because they hardly form any solid solution with themagnetic crystal grain materials described above. Practical examples areSiO_(x), TiO_(x), CrO_(x), AlO_(x), MgO_(x), TaO_(x), YO_(x), TiN_(x),CrN_(x), SiN_(x), AlN_(x), TaN_(x), SiC_(x), TiC_(x), and TaC_(x). x isa number larger than 0.

The material forming the grain boundary region can be either crystallineor amorphous.

The order of stacking of the hard magnetic recording layer and softmagnetic recording layer may also be reversed from that shown in FIG. 1where necessary.

FIG. 2 is a graph showing typical magnetization curves of a comparativeperpendicular magnetic recording layer having only a hard magneticrecording layer, i.e., including no soft magnetic recording layer, andhaving a residual squareness ratio of 1.

This example used a perpendicular magnetic recording medium formed bystacking, on a nonmagnetic glass substrate, a 100-nm thick Co₉₀Zr₅Nb₅film as a soft magnetic underlayer, a 20-nm thick Ru film as anonmagnetic underlayer, and a 20-nm thick (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂film as a hard magnetic recording layer.

FIG. 3 is a graph showing magnetization curves of the perpendicularmagnetic recording layer of the example of the perpendicular magneticrecording medium of the present invention.

This example used a perpendicular magnetic recording medium formed bystacking, on a nonmagnetic glass substrate, a 100-nm thick Co₉₀Zr₅Nb₅film as a soft magnetic underlayer, a 20-nm thick Ru film as anonmagnetic underlayer, a 20-nm thick (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂ filmas a hard magnetic recording layer, and a 4-nm thick Co₅₀Ni₅₀-8 mol %SiO₂ film as a soft magnetic recording layer.

In FIGS. 2 and 3, reference symbols Ms and Mr respectively denote thesaturation magnetization amount and residual magnetization amount, andreference symbols Hc, Hs, and Hn respectively denote the coercive force,saturation magnetic field, and nucleation magnetic field.

The magnetic recording layer of the perpendicular magnetic recordingmedium of the present invention has magnetic characteristics by which amagnetization curve obtained by applying a sufficiently large magneticfield perpendicularly to the film surface to saturate magnetization inthe magnetic recording layer and measuring the relationship between theapplied magnetic field and magnetization amount forms a hysteresis loophaving a residual squareness ratio of 0.95 or less and an irreversiblereversal magnetic field of 0 Oe or less. The residual squareness ratioRs is the ratio Mr/Ms of the saturation magnetization amount Ms to theresidual magnetization amount Mr. The irreversible reversal magneticfield is the magnitude of a magnetic field in which not only themagnetization rotating mechanism reversible to the applied magneticfield but also the magnetization reversing mechanism irreversible to theapplied magnetic field is beginning to cause the magnetization processon the hysteresis loop.

The magnitude of the irreversible reversal magnetic field can be knownby applying a sufficiently large magnetic field in the same manner as inthe hysteresis loop measurement, folding back the loop by reversing thesweeping direction while the magnetic field is swept, and measuring theminor loop.

FIG. 4 is a graph showing schematic magnetization curves for explainingone definition of a term used in the present invention.

When the reversible magnetization rotating mechanism alone causes themagnetization process on the hysteresis loop, as schematically shown inFIG. 4, in a minor loop obtained by a path a→b→a, the foldedmagnetization curve reaches the point a by accurately tracing theoriginal hysteresis loop (major loop). By contrast, when themagnetization process on the hysteresis loop includes the irreversiblemagnetization reversing mechanism, in a minor loop similarly obtained bya path a→c→a, the folded magnetization curve does not trace the majorloop, and a hysteresis appears as shown in FIG. 4. A magnetic field inwhich the minor loop does not overlap the major loop and a hysteresis isbeginning to appear is defined as the irreversible reversal magneticfield Hi.

If the magnetic recording layer is made of a single hard magneticrecording layer or two or more hard magnetic recording layers stronglycoupled by exchange coupling, the magnetic recording layer almostcoherently rotates and/or reverses magnetization in the direction offilm thickness. In this case, the irreversible reversal magnetic fieldHi matches the nucleation magnetic field Hn on the hysteresis loop shownin FIG. 2.

To obtain a high thermal decay resistance in the perpendicular magneticrecording medium, it is generally possible to set the irreversiblereversal magnetic field to 0 or less. In the magnetic recording layerhaving the above structure, this is equivalent to setting the Hn to 0 orless, and this inevitably makes it possible to set the residualsquareness ratio Rs to 1. On the other hand, to improve the writability,it is generally desirable to reduce the coercive force Hc and/or thesaturation magnetic field Hs.

The coercive force Hc and/or the saturation magnetic field Hs can bereduced by reducing the crystal magnetic anisotropic energy of themagnetic crystal grains, or increasing the magnetic interaction betweenthe magnetic crystal grains in the film surface. However, the thermaldecay resistance decreases if the crystal magnetic anisotropic energy ofthe magnetic crystal grains is reduced, and the medium noise increasesif the magnetic interaction between the magnetic crystal grains isincreased. In the magnetic recording layer made of a single hardmagnetic recording layer or two or more hard magnetic recording layersstrongly coupled by exchange coupling, it is necessary to reduce themedium noise and recording magnetic field and increase the thermal decayresistance by using the two methods, i.e., the method of reducing thecrystal magnetic anisotropic energy of the magnetic crystal grains, andthe method of increasing the magnetic interaction between the magneticcrystal grains. However, the increase in recording density achieved bythese methods has already reached its limit.

By contrast, the perpendicular magnetic recording medium of the presentinvention uses, as the perpendicular magnetic recording layer, the softmagnetic recording layer having a weak coercive force and smallsaturation magnetic field together with the hard magnetic recordinglayer having a high thermal decay resistance. In addition, the magneticcharacteristics of the perpendicular magnetic recording layer areadjusted such that the residual squareness ratio is 0.95 or less and thevalue of the irreversible reversal magnetic field is 0 Oe or less on thehysteresis loop perpendicular to the film surface. It is possible byusing this perpendicular magnetic recording layer to reduce the mediumnoise, improve the writability, and increase the thermal decayresistance at the same time.

The present invention achieves the magnetic characteristics by which theirreversible reversal magnetic field is 0 Oe or less, although theresidual squareness ratio is less than 1. This is so presumably becausewhen the exchange coupling force is “moderately” exerted between themagnetic crystal grains in the soft magnetic recording layer and themagnetic crystal grains in the hard magnetic recording layer, the twolayers do not completely coherently reverse magnetization; prior tomagnetization reversal in the hard magnetic recording layer, the softmagnetic recording layer can start reversible magnetization rotationbefore the applied magnetic field reaches the reversal magnetic field inthe hard magnetic recording layer.

This magnetization reversing mechanism will be explained by taking thehysteresis loop shown in FIG. 4 as an example. In the magnetizationprocess from the point a to the point b, only the soft magneticrecording layer having a weak coercive force starts reversiblemagnetization rotation. Then, after the point c (the irreversiblereversal magnetic field), the hard magnetic recording layer having astrong coercive force starts magnetization reversal by receiving theassist of an exchange magnetic field acting between the hard magneticrecording layer and the soft magnetic recording layer during or afterrotation. The soft magnetic recording layer can rotate magnetizationreversibly to the applied magnetic field owing to the “moderate”exchange coupling force described above. Therefore, in the residualstate (equivalent to the recorded state) after the applied magneticfield is removed, the magnetization in the soft magnetic layer canreturn to the same value as in the saturated state. Accordingly, evenwhen the residual squareness ratio on the hysteresis loop is less than1, the irreversible reversal magnetic field is 0 or less, so there isneither an adverse effect on the thermal decay resistance nor a decreasein reproduced output. Furthermore, when reversing the magnetization, thehard magnetic recording layer receives the assist of the exchangemagnetic field acting between the hard magnetic recording layer and softmagnetic recording layer, in addition to the applied magnetic field andits own demagnetizing field. When compared to the case that the hardmagnetic recording layer alone is used, therefore, magnetizationreversal readily occurs with a low applied magnetic field, and thewritability significantly improves.

As described above, when the magnetic recording layer having the hardmagnetic recording layer and soft magnetic recording layer is used, thereversal magnetic field can be reduced without reducing the crystalmagnetic anisotropic energy of the magnetic crystal grains andincreasing the magnetic interaction between the magnetic crystal grainsin the film surface, unlike when the magnetic recording layer made ofthe hard magnetic recording layer alone is used. This makes it possibleto improve the writability, reduce the medium noise, and increase thethermal decay resistance at the same time.

The soft magnetic recording layer alone causes reversible magnetizationrotation first probably because the exchange coupling force ismoderately acting between the hard magnetic recording layer crystalgrains and soft magnetic recording layer crystal grains. If the exchangecoupling force is too strong, the soft magnetic recording layer and hardmagnetic recording layer coherently cause magnetization reversal. Thismakes it impossible to increase the thermal decay resistance and reducethe noise at the same time as in the case that the hard magneticrecording layer alone is used. On the other hand, if the exchangingcoupling is too weak, the soft magnetic recording layer causesirreversible magnetization reversal, and this makes it impossible toincrease the thermal decay resistance and improve the writability at thesame time. The magnetic characteristics and magnetization processmechanisms as described above are achieved by extensively studying thematerials, film thicknesses, film formation methods, and the like of thehard and soft magnetic recording layers, and finding the optimumcondition combinations.

Even when the magnetic recording layer having the soft magneticrecording layer stacked on the hard magnetic recording layer is used, astrong exchange coupling force acts between the hard magnetic crystalgrains and soft magnetic crystal grains if the residual squareness ratioon the hysteresis loop is 1. As described previously, therefore, themagnetizations in the hard magnetic layer and soft magnetic layercoherently behave in the film thickness direction. This makes itimpossible to obtain the effect of reducing the medium noise andrecording magnetic field and increasing the thermal decay resistance atthe same time, such as that of the perpendicular magnetic recordingmedium of the present invention.

Also, if no exchange coupling force acts between the magnetic crystalgrains in the hard magnetic recording layer and the magnetic crystalgrains in the soft magnetic recording layer, the soft magnetic recordinglayer and hard magnetic recording layer cause magnetization reversalcompletely independently of each other. If the soft magnetic recordinglayer causes irreversible magnetization reversal as described above as aresult of this phenomenon, the irreversible reversal magnetic fieldcannot be 0 Oe or less, and the thermal decay resistance decreases. Inaddition, the magnetization in the soft magnetic recording layer freelybehaves in the residual state (i.e., the recorded state), therebyincreasing the medium noise and decreasing the SNR. This makes itimpossible to obtain the effect of the perpendicular magnetic recordingmedium of the present invention as described above.

The coercive force of the magnetic recording layer can be set within therange of 2.5 to 7 kOe, preferably, 3 to 5.5 kOe. If the coercive forceis less than 2.5 kOe, the SNR often decreases. If the coercive forceexceeds 7 kOe, the writability often deteriorates.

The residual squareness ratio of the perpendicular magnetic recordinglayer of the perpendicular magnetic recording medium of the presentinvention can be set within the range of 0.7 to 0.9, preferably, 0.8 to0.9. If the residual squareness ratio is less than 0.7, the writabilityoften deteriorates. If the residual squareness ratio exceeds 0.9, theSNR often decreases.

The irreversible reversal magnetic field can be set within the range of−3.5 to −0.5 kOe, preferably, −3 to −1 kOe. If the irreversible reversalmagnetic field is less than −0.5 kOe, the thermal decay resistance oftendecreases. If the irreversible reversal magnetic field exceeds −3.5 kOe,the writability often deteriorates.

FIG. 5 is a sectional view showing another example of the perpendicularmagnetic recording medium according to the present invention.

As shown in FIG. 5, a perpendicular magnetic recording medium 20 has astacked structure in which a soft magnetic underlayer 2, nonmagneticunderlayer 3, and perpendicular magnetic recording layer 4 aresequentially stacked on a substrate 1. The perpendicular magneticrecording layer 4 has three sequentially stacked layers, i.e., a hardmagnetic recording layer 4-1, nonmagnetic interlayer 4-3, and softmagnetic recording layer 4-2.

The exchange coupling force between the hard magnetic recording layerand soft magnetic recording layer can be further optimally adjusted byforming the thin nonmagnetic interlayer between them.

The film thickness of the nonmagnetic interlayer can be 0.3 to 1.5 nm,preferably, 0.5 to 1 nm. If the nonmagnetic interlayer film thickness isless than 0.3 nm, it is difficult to form a continuous film and obtain anotable effect of controlling the magnetic characteristics. If thenonmagnetic interlayer film thickness exceeds 2 nm, the exchangecoupling significantly weakens, and the soft magnetic recording layeroften causes irreversible magnetization reversal.

The film thickness of the nonmagnetic interlayer can be evaluated by,e.g., sectional TEM observation.

As the nonmagnetic interlayer material, it is possible to use a metal oralloy containing at least one of Pd, Pt, Cu, Ti, Ru, Re, Ir, and Cr.

When the nonmagnetic interlayer has the granular structure, magneticisolation of the hard magnetic recording layer or soft magneticrecording layer stacked on the nonmagnetic interlayer accelerates, sothe SNR can further increase. As the material forming the grain boundaryregion of the nonmagnetic interlayer, a compound such as an oxide,nitride, or carbide can be used. These compounds readily deposit becausethey hardly form any solid solution with the nonmagnetic crystal grainmaterials described above. Practical examples of the material formingthe grain boundary region of the nonmagnetic interlayer are SiO_(x),TiO_(x), CrO_(x), AlO_(x), MgO_(x), TaO_(x), YO_(x), TiN_(x), CrN_(x),SiN_(x), AlN_(x), TaN_(x), SiC_(x), TiC_(x), and TaC_(x).

The material forming the grain boundary region can be either crystallineor amorphous.

As the nonmagnetic underlayer of the perpendicular magnetic recordingmedium of the present invention, it is possible to use a metal or alloycontaining at least one of Ru, Ti, Pt, and Re. For example, the materialis selected from Ru, Ti, Re, and a Pt—Cr alloy. These materials havehigh lattice matching with the magnetic crystal grains in the hardmagnetic recording layer described earlier, and can improve the (0001)orientation of the magnetic crystal grains.

To improve the crystal orientation of the nonmagnetic underlayer, a seedlayer can be formed between the soft magnetic underlayer and nonmagneticunderlayer. Practical examples are Pd, Pt, Ta, Ni—Ta, Ni—Nb, Ni—Zr,Ni—Fe—Cr, and Ni—Fe.

A so-called perpendicular double-layered medium is obtained by forming asoft under layer with high magnetic permeability between the nonmagneticunderlayer and substrate. In this perpendicular double-layered medium,the soft magnetic underlayer is longitudinally orientated. The softmagnetic underlayer horizontally passes a recording magnetic field froma magnetic head, e.g., a single-pole head for magnetizing theperpendicular magnetic recording layer, and returns the magnetic fieldto the magnetic head. That is, the soft magnetic underlayer performspart of the function of the magnetic head. The soft magnetic underlayercan apply a steep, sufficient perpendicular magnetic field to themagnetic field recording layer, thereby increasing therecording/reproduction efficiency.

Examples of the soft magnetic layer as described above are CoZrNb, CoB,CoTaZr, FeSiAl, FeTaC, CoTaC, NiFe, Fe, FeCoB, FeCoN, FeTaN, and CoIr.

The soft magnetic underlayer may also be a multilayered film having twoor more layers. In this case, the materials, compositions, and filmthicknesses of the individual layers can be different. It is alsopossible to form a triple-layered structure by stacking two softmagnetic underlayers with a thin Ru layer being sandwiched between them.

Furthermore, a bias application layer such as a longitudinal hardmagnetic film or antiferromagnetic film can be formed between the softmagnetic underlayer and substrate. The soft magnetic layer easily formsa magnetic domain, and this magnetic domain produces spike noise.Therefore, by applying a magnetic field in one direction along theradial direction of the bias application layer, it is possible to applya biasing magnetic field to the soft magnetic layer formed on the biasapplication layer, thereby preventing the generation of a magnetic wall.It is also possible to finely disperse anisotropy by giving the biasapplication layer a stacked structure, thereby preventing easy formationof a large magnetic domain. Examples of the bias application layermaterial are CoCrPt, CoCrPtB, CoCrPtTa, CoCrPtTaNd, CoSm, CoPt, FePt,CoPtO, CoPtCrO, CoPt—SiO₂, CoCrPt—SiO₂, CoCrPtO—SiO₂, FeMn, IrMn, andPtMn.

As the nonmagnetic substrate, it is possible to use, e.g., a glasssubstrate, an Al-based alloy substrate, an Si single-crystal substratehaving an oxidized surface, ceramics, or plastic. The same effect can beexpected even when the surface of any of these nonmagnetic substrates isplated with an NiP alloy or the like. A protective layer can be formedon the magnetic recording layer. Examples of the protective layer are C,diamond-like carbon (DLC), SiN_(x), SiO_(x), and CN_(x).

As methods of forming the individual layers, it is possible to usevacuum vapor deposition, various sputtering methods, molecular beamepitaxy, ion beam vapor deposition, laser abrasion, and chemical vapordeposition.

FIG. 6 is a partially exploded perspective view of an example of themagnetic recording/reproducing apparatus of the present invention.

In a magnetic recording/reproducing apparatus 70, a rigid magnetic disk61 for recording information according to the present invention ismounted on a spindle 62, and rotated at a predetermined rotational speedby a spindle motor (not shown). A slider 63 on which a recording headfor recording information by accessing the magnetic disk 61 and an MRhead for reproducing information are mounted is fixed to the distal endof a suspension 64 made of a thin leaf spring. The suspension 64 isconnected to one end of an arm 65 having, e.g., a bobbin that holds adriving coil (not shown).

A voice coil motor 66 as a kind of a linear motor is formed at the otherend of the arm 65. The voice coil motor 66 comprises the driving coil(not shown) wound on the bobbin of the arm 65, and a magnetic circuitincluding a permanent magnet and counter yoke opposing each other so asto sandwich the driving coil.

Ball bearings (not shown) formed in the upper and lower portions of afixing shaft 67 hold the arm 65, and the voice coil motor 66 pivots thearm 65. That is, the voice coil motor 66 controls the position of theslider 63 on the magnetic disk 61. Note that reference numeral 68 inFIG. 6 denotes a lid.

EXAMPLES

The present invention will be explained in more detail below by way ofits examples.

Example 1

A 2.5-inch hard disk type nonmagnetic glass substrate (TS-10SXmanufactured by OHARA) was placed in a vacuum chamber of the c-3010sputtering apparatus manufactured by ANELVA.

After the vacuum chamber of the sputtering apparatus was evacuated to1×10⁻⁵ Pa or less, a 100-nm thick Co₉₀Zr₅Nb₅ film as a soft magneticunderlayer, a 20-nm thick Ru film as a nonmagnetic underlayer, a 20-nmthick (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂ film as a hard magnetic recordinglayer, a Co₃₅Ni₆₅-8 mol % SiO₂ film as a soft magnetic recording layer,and a 5-nm thick C film as a protective layer were sequentially formed.The film thickness of the soft magnetic recording layer was changedwithin the range of 1 to 20 nm. After the film formation, the surface ofthe protective layer was coated with a 13-Å thick perfluoropolyether(PFPE) lubricant by dipping, thereby obtaining perpendicular magneticrecording media.

FIG. 7 is a view schematically showing the section of the perpendicularmagnetic recording medium according to Example 1.

As shown in FIG. 7, this perpendicular magnetic recording medium had thesame structure as shown in FIG. 5 except that a protective layer 5 wasformed on a soft magnetic recording layer 4-2.

When forming the films of Co₉₀Zr₅Nb₅, Ru, (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂,Co₃₅Ni₆₅-8 mol % SiO₂, and C, the Ar pressures were respectively 0.7, 5,5, 0.7, and 0.7 Pa, the targets used were respectively Co₉₀Zr₅Nb₅, Ru,(Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂, Co₃₅Ni₆₅-8 mol % SiO₂, and C targets eachhaving a diameter of 164 mm, and the films were formed by DC sputtering.The input power to each target was 1,000 W. The distance between thetarget and substrate was 50 mm, and all the films were formed at roomtemperature.

Comparative Example 1

As a comparative example, a conventional perpendicular magneticrecording medium was manufactured following the same procedures as inExample 1 except that no soft magnetic recording layer was formed.

Comparative Example 2

As a comparative example, a perpendicular magnetic recording medium inwhich a soft magnetic recording layer had no granular structure wasmanufactured as follows.

That is, the perpendicular magnetic recording medium was manufacturedfollowing the same procedures as in Example 1 except that the softmagnetic recording layer was made of Co₃₅Ni₆₅ and had a film thicknessfixed to 4 nm.

Comparative Example 3

As a comparative example, perpendicular magnetic recording media inwhich a soft magnetic recording layer was strongly coupled on a hardmagnetic recording layer by exchange coupling and the residualsquareness ratio on the hysteresis loop was 1 were manufactured asfollows.

A 2.5-inch hard disk type nonmagnetic glass substrate (TS-10SXmanufactured by OHARA) was placed in a vacuum chamber of the c-3010sputtering apparatus manufactured by ANELVA.

After the vacuum chamber of the sputtering apparatus was evacuated to1×10⁻⁵ Pa or less, a 100-nm thick Co₈₉Zr₄Nb₇ film was formed as a softmagnetic underlayer at a substrate temperature of 100° C. After that, ata substrate temperature of 200° C., an 8-nm thick Ni₅₀—Al₅₀ film asnonmagnetic underlayer 1, a 20-nm thick Ru film as nonmagneticunderlayer 2, a 30-nm thick Co₆₂—Cr₂₀—Pt₁₄—B₄ film as a hard magneticrecording layer, a 2-nm thick Co₈₉Zr₄Nb₇ film as a soft magneticrecording layer, and a 5-nm thick C film as a protective layer weresequentially formed on the soft magnetic underlayer. After the filmformation, the surface of the protective layer was coated with a 13-Åthick perfluoropolyether (PFPE) lubricant by dipping, thereby obtainingthe perpendicular magnetic recording media.

When forming the films of Co₈₉Zr₄Nb₇, Ni₅₀—Al₅₀, Ru, Co₆₂—Cr₂₀—Pt₁₄—B₄,Co₈₉Zr₄Nb₇, and C, all the Ar pressures were 0.5 Pa, the targets usedwere respectively Co₈₉Zr₄Nb₇, Ni₅₀—Al₅₀, Ru, Co₆₂—Cr₂₀—Pt₁₄—B₄,Co₈₉Zr₄Nb₇, and C targets each having a diameter of 164 mm, and thefilms were formed by DC sputtering. The input power to all the targetswas 1,000 W. The distance between the target and substrate was 50 mm.

Comparative Example 4

As a comparative example, a perpendicular magnetic recording medium inwhich there was no exchange coupling between a hard magnetic recordinglayer and soft magnetic recording layer and the irreversible reversalmagnetic field was not 0 Oe or less was manufactured as follows.

That is, after films were sequentially formed up to a hard magneticrecording layer following the same procedure as in Comparative Example3, a 10-nm thick Ru film was formed as a nonmagnetic interlayer on thehard magnetic recording layer in order to avoid exchange coupling. Afterthat, a soft magnetic recording layer, protective layer, and lubricantwere sequentially stacked following the same procedure as in Example 3,thereby obtaining the perpendicular magnetic recording medium.

The microstructure of each obtained perpendicular magnetic recordingmedium was evaluated by using a TEM having an acceleration voltage of400 kV.

The hysteresis loop and minor loop perpendicular to the film surface ofthe perpendicular magnetic recording layer of each perpendicularmagnetic recording medium were evaluated by a Kerr effect evaluatingapparatus by using a laser source having a wavelength of 300 nm, underthe conditions that the maximum applied magnetic field was 20 kOe andthe magnetic field sweeping rate was 133 Oe/sec.

Each perpendicular magnetic recording medium was caused to generate Cu—Kα-rays by using the X'pert-MRD X-ray diffraction apparatus manufacturedby Philips, under the conditions that the acceleration voltage was 45 kVand the filament electric current was 40 mA, and the crystal structureand crystal plane orientation were evaluated by the θ-2θ method.

The R/W characteristics of each perpendicular magnetic recording mediumwere evaluated by using a spin stand. As a magnetic head, a combinationof a single-pole head having a recording track width of 0.3 μm and an MRhead having a reproducing track width of 0.2 μm was used.

The measurements were performed in a radial position of 20 mm, i.e., ina fixed position by rotating the disk at 4,200 rpm.

As the medium SNR, the value of the signal-to-noise ratio (SNRm) (S wasthe output at a linear recording density of 119 kfci, and Nm was thevalue of rms (root mean square) at 716 kfci) of a differential waveformobtained through a differentiating circuit.

The medium OW characteristic was evaluated by recording a 119-kfcisignal, overwriting a 250-kfci signal, and measuring the reproducedoutput ratio (attenuation ratio) of the 119-kfci signals before andafter the overwrite.

The medium thermal decay resistance was evaluated in an environment at atemperature of 70° C. by recording a 100-kfci signal, and measuring theratio V1000/V0 of the reproduced output of the 100 kfci signalimmediately after it was recorded to that after the signal was left tostand for 1,000 sec.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. The average grain size of the magneticcrystal grains in Example 1 and Comparative Examples 1 and 2 was 7.8 nm.Also, the results of the composition analysis by TEM-EDX revealed thatthe magnetic crystal grains in Example 1 and Comparative Examples 1 and2 contained Co, Pt, and Cr.

Furthermore, the soft magnetic recording layer of the perpendicularmagnetic recording medium of Example 1 had the granular structure inwhich the grain boundary region surrounded the magnetic crystal grains,similar to the hard magnetic recording layer. The average grain size ofthe magnetic crystal grains was 7.1 nm. On the other hand, the softmagnetic recording layers of the perpendicular magnetic recording mediaof Comparative Examples 2, 3 and 4 were continuous films having nogranular structure.

FIGS. 8, 9, and 10 are graphs showing the relationships between the softmagnetic recording layer film thickness of the perpendicular magneticrecording medium according to Example 1 and the residual squarenessratio Rs, irreversible reversal magnetic field Hi, and coercive force Hcas curves 101, 102, and 103, respectively.

Referring to FIGS. 8 to 10, a point where the soft magnetic recordinglayer film thickness is 0 nm indicates the value of the perpendicularmagnetic recording medium of Comparative Example 1. For comparison,symbols ▪, Δ, and × respectively indicate Comparative Examples 2, 3, and4.

FIGS. 8 to 10 demonstrate that the Rs and Hc monotonously decreased andthe Hi monotonously increased with the soft magnetic recording layerfilm thickness.

FIGS. 11, 12, and 13 are graphs showing the relationships between thesoft magnetic recording layer film thickness of the perpendicularmagnetic recording medium of Example 1 and the SNR, OW, and V1000/V0 bycurves 104, 105, and 106, respectively. Referring to FIGS. 11 to 13, apoint where the soft magnetic recording layer film thickness is 0 nmindicates the value of the perpendicular magnetic recording medium ofComparative Example 1. For comparison, symbols ▪, Δ, and × respectivelyindicate Comparative Examples 2, 3, and 4.

A comparison of FIG. 8 with FIG. 11 reveals that the SNR of Example 1remarkably increased when the Rs was 0.95 or less.

A comparison of FIG. 9 with FIG. 12 shows that the thermal decayresistance was high when the Hi was 0 Oe or less.

Comparing FIG. 10 with FIG. 12 indicates that the 0 W characteristicsignificantly improved when the Hc was 7 kOe or less. Also, comparingFIG. 10 with FIG. 11 shows that the SNR significantly increased when theHc was 2.5 kOe or more.

A comparison of FIGS. 8 and 11 demonstrates that the increase in SNR wasfurther notable when the Rs was 0.9 or less. A comparison of FIGS. 8 and12 indicates that the OW characteristic was favorable when the Rs was0.7 or more.

Comparing FIG. 9 with FIG. 12 reveals that the 0 W characteristicsignificantly improved when the Hi was −3.5 kOe or more. On the otherhand, a comparison of FIG. 9 with FIG. 13 shows that the thermal decayresistance further increased when the Hi was −0.5 kOe or less.

FIG. 8 shows that the Rs of the perpendicular magnetic recording mediumof Comparative Example 3 was 1. This reveals that the hard magneticrecording layer and soft magnetic recording layer strongly coupled witheach other by exchange coupling, and coherently reversed magnetization.Also, the Hi of the perpendicular magnetic recording medium ofComparative Example 4 was larger than 0 Oe.

FIGS. 8, 9, 11, and 12 demonstrate that the perpendicular magneticrecording medium of Example 1 in which the Rs was 0.95 or less and theHi was 0 Oe or less was superior in SNR and OW characteristic to theperpendicular magnetic recording medium of Comparative Example 1.

FIGS. 8, 9, and 11 indicate that the perpendicular magnetic recordingmedium of Example 1 in which the Rs was 0.95 or less and the Hi was 0 Oeor less had an SNR higher than that of the perpendicular magneticrecording medium of Comparative Example 2.

FIGS. 8, 9, 11, and 12 show that the perpendicular magnetic recordingmedium of Example 1 in which the Rs was 0.95 or less and the Hi was 0 Oeor less was superior in SNR and OW characteristic to the perpendicularmagnetic recording medium of Comparative Example 3.

FIGS. 8, 9, 11, and 12 reveal that the perpendicular magnetic recordingmedium of Example 1 in which the Rs was 0.95 or less and the Hi was 0 Oeor less was superior in SNR, OW characteristic, and thermal decayresistance to the perpendicular magnetic recording medium of ComparativeExample 4.

Example 2

Perpendicular magnetic recording media were manufactured following thesame procedures as in Example 1 except that any of Fe—Ni-8 mol % SiO₂,Co—Ni-8 mol % SiO₂, and Fe—Co-8 mol % SiO₂ was used as a soft magneticrecording layer instead of the 20-nm thick (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂film, and the composition ratios of these Fe—Ni, Co—Ni, and Fe—Co alloyswere changed. The soft magnetic recording layer film thickness was fixedto 4 nm.

Each alloy composition was changed by adjusting the target alloycomposition.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of any perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer.

Table 1 below shows the values of the medium SNR, OW, and thermal decayresistance when the soft magnetic recording layers were made ofFe₃₅—Ni₆₅-8 mol % SiO₂, Co₃₅—Ni₆₅-8 mol % SiO₂, Co₆₅—Fe₃₅-8 mol % SiO₂,Fe—SiO₂, and Co—SiO₂.

TABLE 1 Soft magnetic Hi SNR OW recording layer Rs [kOe] Hc [kOe] [dB][dB] V1000/V0 None 1.0 −4.0 8.1 12.0 23 0.998 Co₃₅Ni₆₅—8% SiO₂ 0.95 −3.57.0 16.8 38 0.997 Fe₃₅Ni₆₅—8% SiO₂ 0.91 −2.8 5.4 16.6 40 0.996Fe₅₀Co₅₀—8% SiO₂ 0.87 −2.1 4.8 16.5 43 0.995 Fe—8% SiO₂ 0.84 −1.7 4.216.5 46 0.992 Co—8% SiO₂ 0.89 −2.3 5.0 16.6 42 0.996

Table 1 reveals that when any of Fe, Co, and the Fe—Ni, Co—Ni, and Fe—Coalloys was used as the magnetic crystal grains in the soft magneticrecording layer, the medium SNR and OW significantly improved and thethermal decay resistance also increased even when the soft magneticrecording layer film thickness was as small as 4 nm.

FIGS. 14, 15, and 16 are graphs showing the changes in medium SNR, OW,and thermal decay resistance with respect to the Co composition when thesoft magnetic recording layer was made of CoNi—SiO₂, as curves 107, 108,and 109, respectively. FIGS. 14 to 16 demonstrate that the SNRremarkably increased when the Co composition was 45% to 80%.

Example 3

Perpendicular magnetic recording media were manufactured following thesame procedures as in Example 1 except that a hard magnetic recordinglayer was made of (Co₇₆—Cr₈—Pt₁₆)-8 mol % TiO or (Co₇₆—Cr₈—Pt₁₆)-8 mol %Cr₂O₃, a soft magnetic recording layer was made of any of Co₅₀—Ni₅₀-8mol % TiO, Co₅₀Ni₅₀-8 mol % Cr₂O₃, Co₅₀Ni₅₀-8 mol % Y₂O₃, Co₅₀Ni₅₀-8 mol% MgO, Co₅₀Ni₅₀-8 mol % Al₂O₃, and Co₅₀Ni₅₀-8 mol % Ta₂O₅, and the softmagnetic recording layer film thickness was fixed to 4 nm.

The layers made of (Co₇₆—Cr₈—Pt₁₆)-8 mol % TiO, (Co₇₆—Cr₈—Pt₁₆)-8 mol %Cr₂O₃, Co₅₀—Ni₅₀-8 mol % TiO, Co₅₀Ni₅₀-8 mol % Cr₂O_(3,) Co₅₀Ni₅₀-8 mol% Y₂O₃, Co₅₀Ni₅₀-8 mol % MgO, Co₅₀Ni₅₀-8 mol % Al₂O₃, and Co₅₀Ni₅₀-8 mol% Ta₂O₅ were respectively formed by using targets made of(Co₇₆—Cr₈—Pt₁₆)-8 mol % TiO, (Co₇₆—Cr₈—Pt₁₆)-8 mol % Cr₂O₃, Co₅₀—Ni₅₀-8mol % TiO, Co₅₀Ni₅₀-8 mol % Cr₂O₃, Co₅₀Ni₅₀-8 mol % Y₂O₃, Co₅₀Ni₅₀-8 mol% MgO, Co₅₀Ni₅₀-8 mol % Al₂O₃, and Co₅₀Ni₅₀-8 mol % Ta₂O₅ each having adiameter of 164 mm.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of any perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer.

Table 2 below show the residual squareness ratio Rs, irreversiblereversal magnetic field Hi, coercive force Hc, medium SNR, OW, andthermal decay resistance of each perpendicular magnetic recordingmedium.

TABLE 2 Hard magnetic Soft magnetic recording layer recording layer RsHi [kOe] Hc [kOe] SNR [dB] OW [dB] V1000/V0 CoCrPt—SiO₂ None 1.0 −4.08.1 12.0 23 0.998 CoCrPt—SiO₂ CoNi—SiO₂ 0.82 −2.5 6.1 18.6 41 0.997CoCrPt—SiO₂ CoNi—TiO 0.85 −2.6 6.0 18.9 41 0.998 CoCrPt—SiO₂ CoNi—Cr₂O₃0.86 −2.4 6.1 18.5 42 0.999 CoCrPt—SiO₂ CoNi—MgO 0.86 −2.4 6.2 18.4 430.999 CoCrPt—SiO₂ CoNi—Y₂O₃ 0.88 −2.7 6.0 18.3 42 0.999 CoCrPt—SiO₂CoNi—Al₂O₃ 0.87 −2.7 6.1 18.3 43 0.999 CoCrPt—SiO₂ CoNi—Ta₂O₅ 0.86 −2.86.1 18.4 43 0.997 CoCrPt—TiO None 1.0 −4.1 7.9 12.2 23 0.999 CoCrPt—TiOCoNi—SiO₂ 0.82 −2.6 5.9 18.8 42 0.997 CoCrPt—TiO CoNi—TiO 0.83 −2.6 5.919.0 41 0.997 CoCrPt—TiO CoNi—Cr₂O₃ 0.86 −2.6 6.0 18.7 43 0.998CoCrPt—TiO CoNi—MgO 0.87 −2.5 5.7 18.8 43 0.997 CoCrPt—TiO CoNi—Y₂O₃0.84 −2.4 5.7 18.6 44 0.999 CoCrPt—TiO CoNi—Al₂O₃ 0.84 −2.4 5.6 18.7 440.998 CoCrPt—TiO CoNi—Ta₂O₅ 0.88 −2.7 5.7 18.7 43 0.999 CoCrPt—Cr₂O₃None 1.0 −3.8 7.7 12.0 23 0.998 CoCrPt—Cr₂O₃ CoNi—SiO₂ 0.84 −2.2 5.718.5 45 0.996 CoCrPt—Cr₂O₃ CoNi—TiO 0.86 −2.4 5.6 18.6 44 0.996CoCrPt—Cr₂O₃ CoNi—Cr₂O₃ 0.86 −2.3 5.6 18.4 46 0.997 CoCrPt—Cr₂O₃CoNi—MgO 0.85 −2.3 5.7 18.3 47 0.999 CoCrPt—Cr₂O₃ CoNi—Y₂O₃ 0.88 −2.55.6 18.3 47 0.999 CoCrPt—Cr₂O₃ CoNi—Al₂O₃ 0.87 −2.4 5.7 18.3 47 0.998CoCrPt—Cr₂O₃ CoNi—Ta₂O₅ 0.87 −2.4 5.9 18.4 46 0.999

Table 2 reveal that each medium had a high SNR, a good OWcharacteristic, and a high thermal decay resistance.

Example 4

Perpendicular magnetic recording media were manufactured following thesame procedures as in Example 1 except that a Pd film having a thicknessof 0.1 to 3 nm was formed as a nonmagnetic interlayer between a hardmagnetic recording layer and soft magnetic recording layer, and a 4-nmthick Co₅₀Ni₅₀-8 mol % SiO₂ film was formed as the soft magneticrecording layer. The Pd interlayer was formed by DC sputtering by usinga Pd target 164 mm in diameter at an Ar pressure of 0.7 Pa and an inputpower of 100 W.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of every perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer. On the other hand, none of thenonmagnetic interlayers had the granular structure.

FIGS. 17, 18, 19, 20, 21, and 22 are graphs showing the changes inresidual squareness ratio Rs, irreversible reversal magnetic field Hi,coercive force Hc, medium SNR, OW, and thermal decay resistance withrespect to the Pd interlayer film thickness as curves 110, 111, 112,113, 114, and 115, respectively.

When the Pd interlayer film thickness was 0.5 to 1.5 nm, the SNR and OWsignificantly improved, and the thermal decay resistance was high. Onthe other hand, when the Pd film thickness exceeded 2 nm, theirreversible reversal magnetic field Hi exceeded 0, and the thermaldecay resistance decreased.

Example 5

Perpendicular magnetic recording media were manufactured following thesame procedures as in Example 4 except that a 0.8-nm thick film of anyof Pt, Cu, Ti, Ru, Re, Ir, and Cr was formed, instead of Pd, as anonmagnetic interlayer between a hard magnetic recording layer and softmagnetic recording layer. The Pt, Cu, Ti, Ru, Re, Ir, and Cr interlayerswere formed by DC sputtering by using Pt, Cu, Ti, Ru, Re, Ir, and Crtargets 164 mm in diameter at an Ar pressure of 0.7 Pa and an inputpower of 100 W.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of any perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer. On the other hand, none of thenonmagnetic interlayers had the granular structure.

Table 3 below shows the residual squareness ratio Rs, irreversiblereversal magnetic field Hi, coercive force Hc, medium SNR, OW, andthermal decay resistance of each perpendicular magnetic recordingmedium.

TABLE 3 Nonmagnetic SNR interlayer Rs Hi [kOe] H [kOe] [dB] OW [dB]V1000/V0 None 0.82 −2.5 6.1 18.6 41 0.997 Pd 0.83 −2.8 5.4 20.3 54 0.998Pt 0.82 −2.9 5.3 20.5 53 0.998 Cu 0.84 −2.6 5.3 20.6 51 0.997 Ti 0.84−2.8 5.5 20.5 51 0.997 Re 0.82 −2.6 5.5 20.1 57 0.997 Ru 0.88 −2.6 5.220.0 57 0.999 Ir 0.82 −2.6 5.5 20.0 57 0.997 Cr 0.84 −2.8 5.5 20.1 560.998

Even when the interlayer was changed to Pt, Cu, Ti, Ru, Re, Ir, and Cr,the SNR and OW significantly improved, and the thermal decay resistancewas high.

Example 6

Perpendicular magnetic recording media were manufactured following thesame procedures as in Example 4 except that a 0.8-nm thick film of anyof Pd-8 mol % SiO₂, Pd-8 mol % TiO, Pd-8 mol % Cr₂O₃, Pd—Y₂O₃, Pd—MgO,Pd—Al₂O₃, and Pd—Ta₂O₅ was formed, instead of Pd, as a nonmagneticinterlayer. The Pd-8 mol % SiO₂, Pd-8 mol % TiO, Pd-8 mol % Cr₂O₃,Pd—Y₂O₃, Pd—MgO, Pd—Al₂O₃, and Pd—Ta₂O₃ interlayers were respectivelyformed by DC sputtering by using Pd-8 mol % SiO₂, Pd-8 mol % TiO, Pd-8mol % Cr₂O₃, Pd—Y₂O₃, Pd—MgO, Pd—Al₂O₃, and Pd—Ta₂O₃ targets 164 mm indiameter at an Ar pressure of 0.7 Pa and an input power of 100 W.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of any perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer.

In addition, the nonmagnetic interlayer of any perpendicular magneticrecording medium had the granular structure in which the grain boundaryregion surrounded the magnetic crystal grains, similar to the hardmagnetic recording layer.

Table 4 below shows the residual squareness ratio Rs, irreversiblereversal magnetic field Hi, coercive force Hc, medium SNR, OW, andthermal decay resistance of each perpendicular magnetic recordingmedium.

TABLE 4 Nonmagnetic SNR interlayer Rs H [kOe] Hc [kOe] [dB] OW [dB]V1000/V0 Pd 0.83 −2.8 5.4 20.3 54 0.998 Pd—SiO₂ 0.79 −2.5 5.6 20.9 520.999 Pd—TiO 0.78 −2.4 5.6 21.1 51 0.998 Pd—Cr₂O₃ 0.8 −2.6 5.5 20.8 530.997 Pd—MgO 0.81 −2.6 5.6 20.8 52 0.998 Pd—Y₂O₃ 0.81 −2.7 5.5 20.7 510.997 Pd—Al₂O₃ 0.8 −2.6 5.6 20.8 51 0.998 Pd—Ta₂O₅ 0.79 −2.7 5.5 20.7 530.997

A comparison with Example 4 shows that the SNR increased more notablywhen the nonmagnetic interlayer had the granular structure.

Example 7

Perpendicular magnetic recording media were manufactured following thesame procedures as in Example 4 except that any of Ti, Re, and Pt₅₀Cr₅₀was used instead of Ru as a nonmagnetic underlayer and a 4-nm thickCo₅₀Ni₅₀-8 mol % SiO₂ film was used as a soft magnetic recording layer.The Ti, Re, and Pt₅₀Cr₅₀ underlayers were respectively formed by DCsputtering by using Ti, Re, and PtCr targets 164 mm in diameter at an Arpressure of 5 Pa and an input power of 1,000 W.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of any perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer.

Table 5 below shows the residual squareness ratio Rs, irreversiblereversal magnetic field Hi, coercive force Hc, medium SNR, OW, andthermal decay resistance of each perpendicular magnetic recordingmedium.

TABLE 5 Nonmagnetic undercoating Rs Hi[kOe] Hc[kOe] SNR[dB] Ru 0.82 −2.56.1 18.6 Ti 0.81 −2.2 5.7 18.4 Re 0.8 −2.1 5.6 18.2 PtCr 0.84 −2.6 6.318.8

It was possible to obtain a high SNR and a good 0 W characteristic evenwhen the nonmagnetic underlayer was made of Re, Ti, or Pt₅₀Cr₅₀.

Example 8

A perpendicular magnetic recording medium was manufactured following thesame procedures as in Example 4 except that the stacking positions of ahard magnetic recording layer and soft magnetic recording layer wereswitched and the nonmagnetic interlayer film thickness was fixed to 0.8nm.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of the manufacturedperpendicular magnetic recording medium had the hcp structure and wereorientated in the (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of the manufactured perpendicular magneticrecording medium had the granular structure in which the grain boundaryregion surrounded the magnetic crystal grains. Also, the results of thecomposition analysis by TEM-EDX revealed that the magnetic crystalgrains contained Co, Pt, and Cr.

Furthermore, the soft magnetic recording layer of the manufacturedperpendicular magnetic recording medium had the granular structure inwhich the grain boundary region surrounded the magnetic crystal grains,similar to the hard magnetic recording layer.

Table 6 below shows the residual squareness ratio Rs, irreversiblereversal magnetic field Hi, coercive force Hc, medium SNR, OW, andthermal decay resistance of the perpendicular magnetic recording medium.

TABLE 6 SNR Rs Hi [kOe] Hc [kOe] [dB] OW [dB] V1000/V0 Example 4 0.83−2.8 5.4 20.3 54 0.998 Example 6 0.82 −2.7 5.5 20.4 53 0.998

A comparison with Example 4 reveals that it was possible to obtain ahigh SNR and a good OW characteristic even when the stacking positionsof the hard magnetic recording layer and soft magnetic recording layerwere switched.

Example 9

Perpendicular magnetic recording media in which each of a soft magneticunderlayer, nonmagnetic interlayer, hard magnetic recording layer, andsoft magnetic recording layer was multilayered to have two or morelayers were manufactured as follows.

A 2.5-inch hard disk type nonmagnetic glass substrate (TS-10SXmanufactured by OHARA) was placed in a vacuum chamber of the c-3010sputtering apparatus manufactured by ANELVA.

After the vacuum chamber of the sputtering apparatus was evacuated to1×10⁻⁵ Pa or less, a 50-nm thick Co₉₀Zr₅Nb₅ film as soft magneticunderlayer 1, a 0.8-nm thick Ru layer, and a 50-nm thick Co₉₀Zr₅Nb₅ filmas soft magnetic underlayer 2 were sequentially stacked. On softmagnetic underlayer 2, a 3-nm thick Pt film as nonmagnetic underlayer 1(a seed layer) and a 20-nm thick Ru film as nonmagnetic underlayer 2were sequentially stacked.

On nonmagnetic underlayer 2, a 10-nm thick (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂film as hard magnetic recording layer 1 and a (Co₇₆—Cr₆—Pt₁₈)-8 mol %TiO film as hard magnetic recording layer 2 were sequentially stacked.

After a 1-nm thick Pd film as a nonmagnetic interlayer was stacked onhard magnetic recording layer 2, a 2-nm thick Co₃₅Ni₆₅-8 mol % SiO₂ filmas soft magnetic recording layer 1, a 2-nm thick Fe-8 mol % SiO₂ film assoft magnetic recording layer 2, and a 5-nm thick C film as a protectivelayer were sequentially formed. After the film formation, the surface ofthe protective layer was coated with a 13-Å thick perfluoropolyether(PFPE) lubricant by dipping, thereby obtaining perpendicular magneticrecording media.

When forming the films of Co₉₀Zr₅Nb₅, Pt, Ru, (Co₇₆—Cr₆—Pt₁₈)-8 mol %SiO₂, (Co₇₆—Cr₆—Pt₁₈)-8 mol % TiO, Pd, Co₃₅Ni₆₅-8 mol % SiO₂, Fe-8 mol %SiO₂, and C, the Ar pressures were respectively 0.7, 0.7, 5, 5, 5, 0.7,0.7, 0.7, and 0.7 Pa, the targets used were respectively Co₉₀Zr₅Nb₅, Pt,Ru, (Co₇₆—Cr₆—Pt₁₈)-8 mol % SiO₂, (Co₇₆—Cr₆—Pt₁₈)-8 mol % TiO, Pd,Co₃₅Ni₆₅-8 mol % SiO₂, Fe-8 mol % SiO₂, and C targets each having adiameter of 164 mm, and the films were formed by DC sputtering. Theinput power to each target was 1,000 W. The distance between the targetand substrate was 50 mm, and all the films were formed at roomtemperature.

The results of the XRD evaluation showed that the magnetic crystalgrains in the hard magnetic recording layer of any perpendicularmagnetic recording medium had the hcp structure and were orientated inthe (0001) plane.

The results of the planar TEM observation indicated that the hardmagnetic recording layer of any perpendicular magnetic recording mediumhad the granular structure in which the grain boundary region surroundedthe magnetic crystal grains. Also, the results of the compositionanalysis by TEM-EDX revealed that the magnetic crystal grains containedCo, Pt, and Cr.

Furthermore, the soft magnetic recording layer of any perpendicularmagnetic recording medium had the granular structure in which the grainboundary region surrounded the magnetic crystal grains, similar to thehard magnetic recording layer.

Table 7 below show the medium SNR, OW, and thermal decay resistance ofeach perpendicular magnetic recording medium.

TABLE 7 Hard Hard Soft Soft Soft magnetic magnetic magnetic magneticmagnetic Nonmagnetic Nonmagnetic recording recording recording SNR OWV1000/ Sample underlayer 1 underlayer 2 underlayer 1 underlayer 2 layer1 layer 2 layer 1 [dB] [dB] V0 10-1 CoZrNb None Ru None CoCrPt—SiO2 NoneCoNi—SiO2 20.3 54 0.998 (100 nm) (20 nm) (20 nm) (4 nm) 10-2 CoZrNbCoZrNb Ru None CoCrPt—SiO2 None CoNi—SiO2 20.6 58 0.998 (50 nm) (50 nm)(20 nm) (20 nm) (4 nm) 10-3 CoZrNb CoZrNb Pt Ru CoCrPt—SiO2 NoneCoNi—SiO2 20.9 55 0.998 (50 nm) (50 nm) (6 nm) (20 nm) (20 nm) (4 nm)10-4 CoZrNb CoZrNb Pt Ru CoCrPt—SiO2 CoCrPt—TiO CoNi—SiO2 21.2 54 0.997(50 nm) (50 nm) (6 nm) (20 nm) (10 nm) (10 nm) (4 nm) 10-5 CoZrNb CoZrNbPt Ru CoCrPt—SiO2 CoCrPt—TiO CoNi—SiO2 21.5 59 0.998 (50 nm) (50 nm) (6nm) (20 nm) (10 nm) (10 nm) (2 nm)

Table 7 demonstrate that the SNR and OW characteristic improved wheneach of the soft magnetic underlayer, nonmagnetic underlayer, hardmagnetic recording layer, and soft magnetic recording layer wasmultilayered.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the inventions. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the inventions. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the inventions.

1. A perpendicular magnetic recording medium comprising: a substrate; asoft magnetic underlayer formed on the substrate; a nonmagneticunderlayer formed on the soft magnetic underlayer; and a magneticrecording layer formed on the nonmagnetic underlayer and including ahard magnetic recording layer and a soft magnetic recording layer,wherein each of the hard magnetic recording layer and the soft magneticrecording layer has magnetic crystal grains and a grain boundary regionsurrounding the magnetic crystal grains, the magnetic crystal grains inthe hard magnetic recording layer contain Co and Pt, has an hcpstructure, and are orientated in a (0001) plane, and the magneticrecording layer has a residual squareness ratio of not more than 0.95and an irreversible reversal magnetic field of not more than 0 Oe on amagnetization curve when a magnetic field perpendicular to a substratesurface is applied.
 2. A medium according to claim 1, wherein theresidual squareness ratio on the magnetization curve is 0.7 (inclusive)to 0.9 (inclusive).
 3. A medium according to claim 1, wherein theirreversible reversal magnetic field on the magnetization curve is −3.5to −0.5 kOe.
 4. A medium according to claim 1, wherein the magneticcrystal grains in the soft magnetic recording layer are made of a metalcomponent containing iron and/or cobalt at not less than 35% as a totalatomic ratio.
 5. A medium according to claim 4, wherein the metalcomponent comprises a cobalt-nickel alloy containing 45 to 80 at. % ofcobalt.
 6. A medium according to claim 1, wherein the grain boundaryregion in the hard magnetic recording layer contains a compound selectedfrom the group consisting of an oxide, a nitride, and a carbide, and thecompound contains at least one element selected from the groupconsisting of silicon, titanium, chromium, aluminum, magnesium,tantalum, and yttrium.
 7. A medium according to claim 1, wherein thegrain boundary region in the soft magnetic recording layer contains acompound selected from the group consisting of an oxide, a nitride, anda carbide, and the compound contains at least one element selected fromthe group consisting of silicon, titanium, chromium, aluminum,magnesium, tantalum, and yttrium.
 8. A medium according to claim 1,further comprising a nonmagnetic interlayer formed between the hardmagnetic recording layer and the soft magnetic recording layer.
 9. Amedium according to claim 8, wherein the nonmagnetic interlayer has athickness of 0.3 to 1.5 nm.
 10. A medium according to claim 8, whereinthe nonmagnetic interlayer contains a metal component containing atleast one element selected from the group consisting of palladium,platinum, copper, titanium, ruthenium, rhenium, iridium, and chromium.11. A medium according to claim 10, wherein the nonmagnetic interlayerhas nonmagnetic crystal grains and a grain boundary region surroundingthe nonmagnetic crystal grains, and the grain boundary region is made ofone of an oxide, nitride, and carbide of at least one of silicon,titanium, chromium, aluminum, magnesium, tantalum, and yttrium.
 12. Amedium according to claim 1, wherein the nonmagnetic underlayer containsat least one metal component selected from the group consisting ofruthenium, titanium, and platinum.
 13. A magnetic recording/reproducingapparatus comprising: a perpendicular magnetic recording medium whichincludes a substrate, a soft magnetic underlayer formed on thesubstrate, a nonmagnetic underlayer formed on the soft magneticunderlayer, and a magnetic recording layer formed on the nonmagneticunderlayer and including a hard magnetic recording layer and a softmagnetic recording layer, and in which each of the hard magneticrecording layer and the soft magnetic recording layer has magneticcrystal grains and a grain boundary region surrounding the magneticcrystal grains, the magnetic crystal grains in the hard magneticrecording layer contain Co and Pt, has an hcp structure, and areorientated in a (0001) plane, and the magnetic recording layer has aresidual squareness ratio of not more than 0.95 and an irreversiblereversal magnetic field of not more than 0 Oe on a magnetization curvewhen a magnetic field perpendicular to a substrate surface is applied;and a recording/reproducing head.
 14. An apparatus according to claim13, wherein the residual squareness ratio on the magnetization curve is0.7 (inclusive) to 0.9 (inclusive).
 15. An apparatus according to claim13, wherein the irreversible reversal magnetic field on themagnetization curve is −3.5 to −0.5 kOe.
 16. An apparatus according toclaim 13, wherein the magnetic crystal grains in the soft magneticrecording layer are made of a metal component containing iron and/orcobalt at not less than 35% as a total atomic ratio.
 17. An apparatusaccording to claim 16, wherein the metal component comprises acobalt-nickel alloy containing 45 to 80 at. % of cobalt.
 18. Anapparatus according to claim 13, wherein the grain boundary region inthe hard magnetic recording layer contains a compound selected from thegroup consisting of an oxide, a nitride, and a carbide, and the compoundcontains at least one element selected from the group consisting ofsilicon, titanium, chromium, aluminum, magnesium, tantalum, and yttrium.19. An apparatus according to claim 13, wherein the grain boundaryregion in the soft magnetic recording layer contains a compound selectedfrom the group consisting of an oxide, a nitride, and a carbide, and thecompound contains at least one element selected from the groupconsisting of silicon, titanium, chromium, aluminum, magnesium,tantalum, and yttrium.
 20. An apparatus according to claim 13, furthercomprising a nonmagnetic interlayer formed between the hard magneticrecording layer and the soft magnetic recording layer.
 21. An apparatusaccording to claim 20, wherein the nonmagnetic interlayer has athickness of 0.3 to 1.5 nm.
 22. An apparatus according to claim 20,wherein the nonmagnetic interlayer contains a metal component containingat least one element selected from the group consisting of palladium,platinum, copper, titanium, ruthenium, rhenium, iridium, and chromium.23. An apparatus according to claim 20, wherein the nonmagneticinterlayer has nonmagnetic crystal grains and a grain boundary regionsurrounding the nonmagnetic crystal grains, and the grain boundaryregion is made of one of an oxide, nitride, and carbide of at least oneof silicon, titanium, chromium, aluminum, magnesium, tantalum, andyttrium.
 24. An apparatus according to claim 13, wherein the nonmagneticunderlayer contains at least one metal component selected from the groupconsisting of ruthenium, titanium, and platinum.